I believe he would be an effective therapist, as he has the ability to point out pragmatic and no-nonsense solutions to relationship issues.
That being said, Dr. Phil is primarily an entertainer. His current focus has more to do with ratings than mental health. If you watch closely, you'll notice most all his guests tend to have issues on the controversial and/or outrageous side.. This is not an accident. He nearly always drives the show in a controversial direction, the plays devil's advocate to both parties. Chances are you wouldn't see this if you were watching normal everyday therapy sessions. That being said, Dr. Phil does is a richly talented entertainer, much like Dr. Oz.